Wireless local area networks (WLANs), such as a wireless medium in a communication network using Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), are founded on the principles of collision avoidance. Such networks may also conform to a communication standard such as a communication protocol of 802.11 type e.g. Medium Access Control (MAC). The collision avoidance approach aims to separate concurrent transmissions in space and time. Packets of e.g. data are corrupted due to collision or channel fading. The transmitter remains unaware of the corruption and continues to transmit the entire packet unnecessarily. Eventually, based on the absence of an acknowledgment from the receiver, the transmitter infers packet loss, and prepares for retransmission.
The IEEE 802.11 standard defines the way WLANs must work at the physical and medium access control (MAC) level. Typically, the 802.11 MAC (Medium Access Control) relies on a contention-based mechanism based on a technique called “Carrier Sense Multiple Access with Collision Avoidance” (CSMA/CA). The standard 802.11 medium access protocol is essentially to wait for the medium to become idle.
Carrier sensing is performed by both physical and virtual mechanisms. The virtual carrier sensing is achieved by transmitting control packets to reserve the medium prior to transmission of data packets. The transmitter attempts to sense an idle medium for at least a DIFS (distributed interframe spacing) duration of time. If the medium is sensed busy, the transmitter waits until it becomes idle and then starts a countdown back-off timer set to expire after a number of slot times, chosen randomly between [0,CW] (i.e. the contention window CW being an integer value).
More precisely, the period of back-off timer countdown is called the ‘back-off procedure’, and is implemented as follows:    Upon starting the back-off process, prior to data transmission, a station initializes its back-off time counter to a ‘random value’. The back-off time counter is decremented once every time interval for so long as the channel is sensed to be idle (e.g. with reference to FIG. 1, count down starting from T0), halted (or frozen) when a transmission is detected on the channel (e.g. with reference to FIG. 1, count down stopping at T1), and reactivated when the channel is sensed idle again (e.g. with reference to FIG. 1, T2). When its back-off time counter reaches zero, a station transmits a DATA message (or ready-to-send (RTS) message as explained hereafter) which contains the address of the receiver and the duration for which the medium is to be reserved for that message. A collision occurs when two or more stations start transmission simultaneously (e.g. when their own back-off counter has reached zero around the same time). Ideally, the same number of slots is present among all nodes forming the 802.11 cell during the back-off countdown procedure.
Since the CSMA/CA protocol does not rely on the capability of the stations to detect a collision by hearing their own transmission, a positive acknowledgement (ACK) is transmitted by the destination station to signal the successful packet reception. The ACK is immediately transmitted at the end of the packet, after a period of time called Short InterFrame Space (SIFS). If the transmitting station does not receive the ACK within a specified ACK Timeout, or it detects the transmission of a different packet on the channel, it reschedules the packet transmission according to the given back-off rules.
Collision Avoidance is more specifically enhanced by a four-way handshaking mechanism called RTS/CTS (request-to-send/clear-to-send) exchange, which is a recommended option of the 802.11 standard, and will be detailed with reference to FIG. 1.
US patent application 2009/0141738 A1 discloses a time slot system where a back-off counter value determination engine selects an unreserved time slot for a next transmission and a medium access engine for initiating a current transmission. This document uses a reservation-based distributed collision avoidance channel access in a wireless area network. By advertising the future channel access parameters in advance, nodes reduce the number of collisions. However, any extra functionality devised to maintain the reservation procedure adds overhead to the functioning of the device and consumes more bandwidth, both situations being very undesirable.
The present inventors have therefore envisaged using a collaborative medium access scheme for several nodes at a time, those nodes pertaining to a group of peer nodes, also called collaborative nodes. It is envisaged that there will also exist nodes outside of the collaborative group, and that communication between nodes inside and nodes outside the group may be required. Nodes outside the group may be referred to as legacy nodes. A legacy environment typically describes a situation where nodes are independent and do not interact or cooperate with each other, as opposed to the collaborative group of nodes. A peer node may request access to the shared 802.11 type medium according to the 802.11 legacy protocol, and upon grant of access, the node may communicate with one or more peer nodes according to a collaborative protocol during the reserved talk time. Thus, if the back-off count reduces to zero for one peer node among the group, said node reserves medium access (through classical RTS/CTS scheme) for the group and lets the group share this granted 802.11 timeslot.
A collaborative node can either communicate inside the group via the collaborative access scheme, or it communicates with legacy nodes outside the collaborative group using the 802.11 legacy protocol: thus no mixed mode is supported which would allow a collaborative node to communicate indifferently with peer nodes and legacy node. Indeed, the collaborative group has no mean to know the communication timeslot which is going to be granted to a collaborative node for a legacy communication, which prevents a collaborative group communication to take place (as example a “time division multiple access”, aka TDMA, communication). Therefore, a collaborative medium access scheme, like for example a distributed back-off mechanism, does not allow communicating with legacy nodes, except by leaving the collaborative sub-network, then eventually joining again after completing the data transmission with the legacy.
Thus aforementioned problems are limiting the optimal functioning of a wireless network.